1. Field of the Invention
The present invention relates in general to the control of switchgear mechanisms which supply power to process control apparatuses, and more specifically to the control of reactor trips in a pressurized light water nuclear reactor, including sensing the condition of power supplied to devices which position control rods in the reactor.
2. Description of the Related Art
The most important step in the shutting down of a nuclear reactor is the insertion of control rods into the reator core. Conventionally, the control rods in a pressurized light water nuclear reactor are positioned by spring-loaded mechanical grippers which hold the control rods only while supplied with electrical power. When the power is cut off, the mechanical grippers release the control rods which fall into the reactor under the force of gravity.
A switchgear mechanism is connected to the power supply line for the rod positioning grippers to interrupt the supply of power when an unsafe condition is sensed by the reactor control and protection systems. Typically, the rod positioners are supplied with alternating current (AC) power at 480 volts. Conventional switchgear mechanisms utilize low resistance, direct current (DC) shunt trip coils which respond quickly but require that the power supplied thereto be cut off shortly after activation of the switchgear to prevent overheating and resultant burnout of the shunt trip coils. Conventionally the trip coil is powered by a DC battery which supplies, e.g., 125 volts under the control of a manually or automatically activated switch. Auxiliary contacts on the switchgear are connected in series with the battery and trip coil to break the circuit between the battery and trip coil after the shunt trip coil is activated. Thus, de-energization of the shunt trip coil conventionally depends upon the mechanical operation of the switchgear, which can be tested only by activation of the switchgear.